In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Tungsten carbide-based composites, include small grains (μm-scale) of at least one hard phase in a binder phase. In tungsten carbide based hard metals, the hard phase tungsten carbide (WC) is always present. In addition, other metal carbide compounds with a general composition (Ti,Nb,Ta,W)C may also be included, as well as metal carbonitrides, e.g., Ti(C,N). The binder phase usually contains cobalt (Co). Other binder phase compositions may also be used, e.g., combinations of Co, Ni, and Fe, or Ni and Fe.
One route in the industrial production of tungsten carbide based hard metals includes blending of appropriate proportions of raw materials and additives with alcohol (often ethanol) or water or a mixture thereof. The blend is then wet milled into a slurry. The purpose of the wet milling is to obtain deagglomeration of the raw materials, to homogenize the distribution of all constituents, and, to some extent, to disintegrate individual raw material grains. After completion of the milling process, the obtained slurry is dried and granulated, e.g., in a spray drier. The obtained granulate is then used for uniaxial pressing of green bodies, or for the production of pellets for injection moulding or extrusion.
The processes mentioned above require the use of ethanol and energy, as well as equipment and manpower time. For economical and ecological reasons, it would be desirable to reduce these usages.
The paper “Dispersing WC-Co powders in aqueous media with polyethylenimine” (E. Laarz and L. Bergström, International Journal of Refractory Metals & Hard Materials, 18, 2000, p 281-286) describes the effect of a cationic polyelectrolyte-polyethylenimine, PEI, in slurries of tungsten carbide and cobalt in water. At concentrations above 0.3% with respect to dry powder weight, PEI acts as a dispersant.
EP-A-1153652 relates to the use of PEI in the preparation of dispersed suspensions of WC and Co in water or water-ethanol mixtures. PEI was reported to have a dispersing effect at concentrations above 0.3% in relation to raw material weight in water based slurries with 3.5 wt % polyethylene glycol (PEG) present. In water-ethanol mixtures, the lowest PEI concentration reported to have a dispersing effect was 0.3 wt % in a slurry of WC, TaC, TiC, TiN, and Co powders in a mixture of 90 wt % ethanol and 10 wt % water. A concentration range of 0.1-10 wt % of a polyethylenimine-based polyelectrolyte was claimed.